The present invention relates to a power distribution apparatus for a four-wheel drive vehicle. The power distribution apparatus efficiently distributes drive force from a transmission, which is connected to a transversely laid out engine, to front and rear wheels via a hydraulic multi-plate clutch that is disposed together with required constituent components thereof efficiently.
In a conventional power distribution apparatus for a four-wheel drive vehicle, a distribution of a predetermined power to front and rear wheels is usually accomplished by controlling a clamping force of a hydraulic multi-plate clutch as a transfer clutch. For controlling the clamping force of the hydraulic multi-plate clutch, it is required to have at least an oil pressure control portion having a control valve for varying the clamping force of the hydraulic multi-plate clutch, and an oil pressure source for generating an oil pressure for the hydraulic multi-plate clutch.
As the most popular case of an arrangement of an engine, the engine is arranged on a front side of a vehicle body in order to secure a passenger space as wide as possible in the vehicle body. In addition, as the arrangement of the engine, a transversal arrangement is adopted in such a manner that an output shaft of the engine is disposed in parallel with a wheel shaft. This construction also applies to a four-wheel drive vehicle.
As a four-wheel drive vehicle having a transversely laid out front engine which is described above, for example, Japanese Unexamined Patent Publication No. Hei. 6-92156 discloses a construction in which a hydraulic multi-plate clutch distributes to the rear wheel side, a predetermined drive force inputted into a differential from a transmission. The hydraulic multi-plate clutch is mounted on a front drive shaft beside a differential for the front wheels.
In addition, Japanese Patent Unexamined Publication No. Hei 6-107019 discloses a four-wheel drive vehicle having a hydraulic multi-plate clutch. The hydraulic multi-plate clutch that distributes a drive force to the rear wheel side is provided between a rear end of a gear shaft extending rearwards from one of a pair of direction changing gears for changing a drive force transmission direction to the rear wheel side and a distal end of a propeller shaft.
However, in a case where the hydraulic multi-plate clutch is disposed on the axis of the front drive shaft as described in the former example of the conventional power distribution apparatus, a large modification needs to be made to a two-wheel drive portion including the front differential. This makes it difficult to use a transaxle for a two-wheel drive vehicle commonly for a four-wheel drive vehicle.
In addition, in a case where the hydraulic multi-plate clutch is provided between the rear end of the gear shaft extending rearward and the distal end of the propeller shaft as done in the latter example of the conventional power distribution apparatus, the diameter of the clutch portion of the transfer unit is increased longitudinally and this leads to the enlargement of the transfer unit. In addition, this enlarged portion of the clutch portion typically runs across the vehicle body chassis structure such as a steering gearbox and a cross member thereabove or therebelow. Therefore in order to maintain a good positional relationship between those members and the transfer unit, the steering gearbox needs to be re-located and the cross member needs to be curved, thus making the vehicle structure complex. In addition, allowing the enlarged hydraulic clutch portion to run across the steering gearbox and the cross member thereabove or therebelow unfavorably reduces the space within the passenger compartment and lowers the minimum road clearance of the vehicle. Furthermore, since the oil pressure control mechanism portion and the oil pressure source is provided in the hydraulic multi-plate clutch as described before, these members need to be disposed efficiently within a limited space. Because of these conventional arrangements, consideration is required in maintaining the oil-tightness of the difference in nature between the operating fluid for operating the hydraulic multi-plate clutch and the gear fluid within the transfer unit. In addition, consideration is also required to be given to the productivity of transmissions and transfer units, as well as the easy access to the transmission and the transfer unit for servicing them for maintenance after they are disposed on the vehicle.
The present invention was made in view of the above problems and objects thereof are to provide a power distribution apparatus for a four-wheel drive vehicle which is compact in size, which is subject to no limitations as to the apparatus"" position relative to the vehicle body structure such as the steering gearbox and the cross member, which can secure with ease the oil-tightness for hydraulic fluid for operating the hydraulic multi-plate clutch, which is superior in providing good productivity of transmissions and transfer units and serviceability thereof for maintenance after they are installed, and which is easy to be installed on the vehicle.
With a view to attain the above objects, according to a first aspect of the present invention, there is provided a power distribution apparatus for a four-wheel drive vehicle comprising: a transversely laid out front engine outputting a drive force via a transmission coupled thereto; a transfer unit including: a pair of direction changing gears for changing a transmission direction of the drive force to a rear direction of the vehicle; a first gear provided on a gear shaft extending rearward from one of the pair of direction changing gears; a second gear adapted to mesh with the first gear so as to transfer the drive force transmission direction to be in parallel with the rear direction; an output shaft adapted to rotatably pass through an axial center of a gear shaft of the second gear so as to output the drive force to the rear direction of the vehicle; a hydraulic multi-plate clutch for variably transmitting torque between the gear shaft of the second gear and the output shaft; and an oil pressure control mechanism integrally incorporating at least an oil pressure source for generating a hydraulic oil pressure for the hydraulic multi-plate clutch and an oil pressure control portion for controlling the clamping force of the hydraulic multi-plate clutch; wherein the hydraulic multi-plate clutch is disposed on the axial center of the second gear at a position closer to the engine than the second gear, the oil pressure control mechanism portion is disposed close to a side of the hydraulic multi-plate clutch, and the drive force is distributed to rear wheels via the hydraulic multi-plate clutch of the transfer unit disposed to the rear of the engine.
According to a second aspect of the present invention, there is provided a power distribution apparatus for a four-wheel drive vehicle as set forth in the first aspect of the present invention, wherein the hydraulic multi-plate clutch and the oil pressure control mechanism portion are accommodated and disposed independently and oil-tightly relative to a gear mechanism portion.
According to a third aspect of the present invention, there is provided a power distribution apparatus for a four-wheel drive vehicle as set forth in the first or second aspect of the present invention, wherein an accommodation chamber for accommodating therein the hydraulic multi-plate clutch has an opening in one side thereof, and where a member for closing the opening is the oil pressure control mechanism portion.
Namely, in the power distribution apparatus for a four-wheel drive vehicle according to the first aspect of the present invention, the transmission direction of the drive force from the transmission coupled to the transversely laid out front engine is changed to the rear by way of the pair of direction changing gears. The drive force is transmitted from the first gear, which is provided on the gear shaft extending to the rear from one of the pair of direction changing gears, to the second gear, so that the transmission direction of the drive force is transfered in parallel. Then, when the control valve in the oil pressure control portion is actuated so that the predetermined oil pressure is applied to the hydraulic multi-plate clutch by the hydraulic fluid, the hydraulic multi-plate clutch is clamped. The drive force corresponding to the clamping force of the clutch is then transmitted from the gear shaft of the second gear to the output shaft. Therefore, the transmitted drive force is outputted to the rear wheel side. Here, since the hydraulic multi-plate clutch is disposed between the second gear and the engine, the transfer unit can be constructed and laid out in a compact manner in both transverse and longitudinal directions. In addition, the hydraulic multi-plate clutch is subject to little positional limitation relative to the vehicle body structure such as the steering gearbox and the cross member, thus providing superior mounting properties. Furthermore, because the hydraulic multi-plate clutch is disposed between the second gear and the engine, the oil pressure control portion and the oil pressure source may be integrally coupled together disposed at the substantially horizontal side position on the hydraulic multi-plate clutch as the oil pressure control mechanism portion. This allows the oil pressure control mechanism portion to be disposed closer to the relatively spacious rear of the engine, thereby solving the problem that the oil pressure control mechanism portion occupies the space on the transmission side. In addition, since the oil pressure control mechanism portion is constructed by integrally coupling the oil pressure control portion and the oil pressure source, as with the power distribution apparatus for a four-wheel drive vehicle according to the second aspect of the present invention, the hydraulic multi-plate clutch and the oil pressure control mechanism portion are accommodated and disposed independently and in an oil-tight manner relative to the gear mechanism portion, whereby the oil-tightness can easily be secured relative to the other transmission oil and the like. Furthermore, since the oil pressure control mechanism portion is constructed by integrally coupling the oil pressure control portion and the oil pressure source, there is no need to form oil paths or the like on the transmission side. Therefore, the transmission can be commonly used for both two-wheel drive vehicles and four-wheel drive vehicles. Moreover, the productivity of transfer units and the serviceability thereof for maintenance after installation can be improved.
According to the third aspect of the present invention, there is provided the power distribution apparatus for a four-wheel drive vehicle as set forth in the first or second aspect of the present invention, wherein the transfer unit further includes an accommodation chamber for accommodating the hydraulic multi-plate clutch therein, the accommodation chamber has an opening in one side thereof, and where the opening is closed by the oil pressure control mechanism portion. Thus, the power distribution apparatus can be made more space efficient and the serviceability for maintenance can also be improved.